Love not finding us equal, equalizes us, not finding us united, unites us. Now the Father and the Son finding Themselves not only equal and united, but even one same God, one same goodness, one same essence and one same unity, how much must They needs love one another. But this love does not act like the love which intellectual creatures have amongst themselves, or towards their Creator; for created love is exercised by many and various movements, aspirations, unions and joinings which immediately succeed one another, and make a continuation of love with a grateful vicissitude of spiritual movements, but the divine love of the eternal Father towards His Son is practised in one only spiration (souspir) mutually from Them both, Who in this sort remain united and joined together. Yes, Theotimus; for the goodness of the Father and Son being but one sole most perfectly singular goodness, common to Them both, the love of this goodness can be but one only love; for though there be two lovers, to wit, the Father and the Son, yet seeing it is only Their most singular goodness common to Them both which is loved, and Their most unique will which loves, it is therefore but one love exercised by one amorous spiration. The Father breathes this love and so does the Son; but because the Father only breathes this love by means of the same will and for the same goodness which is equally and singularly in Him and His Son: the Son again only breathes this spiration of love for this same goodness and by this same will, -- therefore this spiration of love is but one spiration, or one only spirit breathed out by two breathers.
And because the Father and the Son Who breathe, have an infinite essence and will by which They breathe, and because the goodness for which They breathe is infinite, it is impossible Their breathing should not be infinite; and forasmuch as it cannot be infinite without being God, therefore this Spirit breathed from the Father and the Son is true God: and since there neither is, nor can be, more than one only God, He is one only true God with the Father and the Son. Moreover, as this love is an act which proceeds mutually from the Father and the Son, it can neither be the Father, nor the Son, from whom it proceeds, though it has the same goodness and substance of the Father and the Son, but must necessarily be a third person, Who with the Father and the Son is one only God. And because this love is produced by manner of breathings or spirations, it is called the Holy Spirit.
Now, Theotimus, King David, describing the sweetness of the friendship of God's servants, cries out: Behold how good and how pleasant it is for brethren to dwell together in unity: like the precious ointment on the head, that ran down upon the beard, the beard of Aaron, which ran down to the skirt of his garment: as the dew of Hermon, which descendeth upon Mount Sion. 
But, O God! if human friendship be so agreeably lovely, and spread so delicious an odour on them that contemplate it, what shall it be, my well-beloved Theotimus, to behold the sacred exercise of mutual love between the eternal Father and the Son. S. Gregory Nazianzen recounts that the incomparable love which existed between him and S. Basil the Great was famous all through Greece, and Tertullian testifies, that the Pagans admired the more than brotherly love which reigned amongst the primitive Christians. Oh! with what celebration and solemnity, with what praises and benedictions, should be kept, with what admirations should be honoured and loved, the eternal and sovereign friendship of the Father and the Son! What is there to be loved and desired if friendship is not? And if friendship is to be loved and desired, what friendship can be so in comparison with that infinite friendship which is between the Father and the Son, and Which is one same most sole God with them? Our heart, Theotimus, will sink lost in love, through admiration of the beauty and sweetness of the love, that this eternal Father and this incomprehensible Son practise divinely and eternally.
 Psalm 132.